The present invention relates generally to ultrasound imaging systems, and more specifically to ultrasound imaging systems which provide improved visualization of contrast agents.
Ultrasound imaging systems usually are operated in a fashion to produce real-time moving images of a subject being scanned. These moving images are acquired as discrete static images, but at a high enough frame rate (typically 20-30 frames/sec) to present the illusion of a continuously moving image. Commercial ultrasound systems have also included triggered acquisition modes. In these modes, an ultrasound image frame is acquired at a specified point in each cardiac cycle, as measured for example by a delay from the R-wave of an ECG waveform. Typically the ultrasound system is quiescent between acquisition of successive triggered frames, neither transmitting nor receiving, and the system display is static, showing the last triggered frame. For example, an ultrasound system can be programmed to generate a triggered frame at 100 ms after each R-wave. At typical human heart rates of 60-120 beats/minute, this results in the image being updated at 1-2 frames/second, rather than the 20 or more frames/sec that might be possible if scanning were continuous. In other variants, triggered frames may be acquired only on selected beats (e.g., 150 ms after every 3rd R-wave), or multiple frames may be acquired per beat (e.g., 100, 150, and 250 ms after every 2nd R-wave).
One application where gated imaging modes are useful is imaging of ultrasound contrast agents. Contrast agents are injected into the bloodstream to increase the brightness of blood and blood-perfused tissues.
However, these contrast agents (which are typically composed of stabilized gas microbubbles a few microns in diameter) are fragile and easily degraded (destroyed or altered) by the ultrasound pulses used to image them. A first ultrasound frame may show the contrast agent well, but subsequent frames often show less and less signal as the contrast agent is destroyed.
Thomas Porter and other researchers have demonstrated that gated imaging may be used to advantage where bubble destruction is an issue. A single image frame is acquired every cardiac cycle (or every few cardiac cycles). During the interval between frame acquisition, while the ultrasound transmitters are inactive, fresh contrast agent circulates into the tissues and vessels being imaged. Thus, if the interval between successive triggered frames is sufficiently long, a second acquired frame presents a signal that is as strong as the first. Some researchers have proposed that bubbles are not destroyed by ultrasound, but are altered in some way; during the interval between frames, the bubbles may recover in some way. In this case, the effect is the same: after an interval without transmission, the image returns to its initial brightness.
Another method used to reduce bubble destruction is to transmit at a reduced ultrasound intensity. This reduces bubble destruction at the cost of a reduced signal-to-noise ratio.
Another property of contrast agents that should be mentioned is non-linear scattering. Many contrast agents, when insonified with an acoustic pulse centered at one frequency, reflect or radiate ultrasound containing components at harmonics of the insonifying frequency as well as at the insonifying frequency. This property has been used to advantage in distinguishing contrast agents from normal tissues, which do not tend to scatter non-linearly. U.S. Pat. No. 5,255,683 (Monaghan), U.S. Pat. No. 5,410,516 (Uhlendorf), U.S. Pat. No. 5,456,257 (Johnson) and U.S. Pat. No. 5,577,505 (Brock-Fisher) disclose techniques for imaging non-linear scattering from tissues, and several ultrasound manufacturers are known to be developing second harmonic imaging capability (that is, forming an image from energy scattered at a harmonic multiple of twice the insonifying frequency). Harmonic imaging suffers from the same bubble destruction effects as does conventional fundamental imaging, and the same techniques of gating and reduced transmit power may be used.
Conventional gated imaging techniques require a user to hold an ultrasound probe in a fixed location for as long as several heartbeats without any visual feedback from the image, which statically shows the previously acquired frame. Furthermore, static images of dynamic structures such as the heart may be difficult to interpret and may contain less diagnostic information than moving images. Reducing transmit power reduces bubble destruction, but makes the image noisier and ultimately limits the penetration depth (the maximum depth that may successfully be scanned). Transmit power reductions of 15 dB or more (below the maximum attainable by a typical diagnostic system under FDA limitations) may be required in order to avoid destroying bubbles when imaging perfusion of tissues. Transmit power reduction may be particularly disadvantageous in harmonic imaging. The harmonic component of the scattered and received signal is typically much smaller than the fundamental component (only a small fraction of the incident acoustic energy is converted to higher harmonic frequencies), while the noise floor remains roughly constant. Further, because of the non-linearity inherent in generating higher-order harmonics, a given reduction in transmit power results in an even greater reduction in the harmonic signal strength. For example, a 3 dB transmit power reduction may result in roughly a 6 dB decrease in the level of the second harmonic signal.